A problem often encountered in the production of natural gas from underground reservoirs is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoirs as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation.
One conventional method of removing the nitrogen contaminant from the natural gas is to pass a stream containing nitrogen and methane to a nitrogen rejection unit (NRU) comprising double cryogenic rectification columns wherein the nitrogen and methane are separated.
Although this conventional method for separating nitrogen and methane has worked reasonably well, two possible problems, both related to the nature of rectification, have heretofore acted as a detriment to the efficiency of the method.
One problem relates to the fact that rectification relies on mass transfer of components in different concentrations in countercurrent liquid-vapor flow. The efficiency of such mass transfer is hindered by higher pressures and thus the rectification is best conducted at low pressures consistent with required concentration driving forces and temperature considerations well known to those skilled in the art. Such lower pressure rectification produces methane at the low pressure. Although the methane product from the low pressure column is liquid pumped to a higher pressure prior to rewarming, that pumping is limited by process refrigeration requirements. Typically, the methane product pressure level as it emerges from the double column NRU is considerably lower than the feed pressure level. Since it is often desired to have product methane at higher pressure, for example for introduction into a high pressure natural gas pipeline, the methane product must be compressed to a higher pressure than that at which it is when it emerges from the double column NRU. This compression involves considerable expense. It is therefore desirable to have a nitrogen-methane separation process employing cryogenic rectification wherein some methane product can be produced at a pressure which exceeds that of the methane as it emerges from the double column NRU, consequently reducing the amount of methane compression required.
Another problem relates to the fact that the efficiency of the double column cryogenic rectification is hindered at low concentrations of the more volatile component as this reduces the quality of the available reflux for the top of the low pressure column. In the case of a nitrogen methane mixture, the efficiency of the NRU is significantly reduced when the NRU feed has a nitrogen concentration of less than about 35 mole percent. This problem has been addressed by recycling a portion of the nitrogen stream from the NRU separation back to the natural gas feed stream, thus keeping the nitrogen concentration high enough for effective separation. However, this method has two disadvantages. First, use of a nitrogen recycle in this manner increases the NRU plant size requirements. Second, this process leads to significantly increased power requirements, since relatively pure nitrogen from the exit stream must be separated over again from the natural gas feed.
A recent significant advancement in a double-column NRU process is described in U.S. Pat. No. 4,415,345 -Swallow. In this process, a portion of the product nitrogen stream from the low pressure column is rewarmed to ambient temperature, compressed to the pressure level of the high pressure column, and then cooled against the rewarming low pressure nitrogen. This nitrogen stream is then condensed in the low pressure column reboiler along with the nitrogen vapor from the high pressure column. By supplementing the amount of nitrogen condensed in this manner, which is often referred to as a nitrogen heat pump, additional nitrogen reflux is available to the low pressure column, thereby permitting a higher percentage recovery of inlet methane. This process has the advantage over the previous state of the art in that a reduction in capital and operating costs is achieved. However, process equipment such as distillation columns and heat exchangers must still be sized for the additional recirculation of nitrogen and a separate nitrogen gas compressor is still required. It is therefore desirable to have a nitrogen-methane separation process employing double column cryogenic rectification which can operate more efficiently at lower available nitrogen concentrations without the need for nitrogen recycle or a nitrogen heat pump.
Another problem often encountered in the cryogenic separation of nitrogen and methane in a stream from a natural gas reservoir is a high concentration of carbon dioxide in the nitrogen-methane feed. Since carbon dioxide freezes at a relatively high temperature, carbon dioxide must be removed from the feed prior to cryogenic processing so as to be at a level which does not exceed solubility limits. As can be appreciated the higher is the concentration of carbon dioxide in the original feed, the more expensive, from both a capital and operating cost standpoint, is the carbon dioxide removal step. It would therefore be desirable to have a nitrogen methane separation process which can tolerate a significantly higher carbon dioxide concentration in the nitrogen-methane feed without the need for extensive and expensive carbon dioxide removal steps.
It is therefore an object of this invention to provide an improved process for separating nitrogen and methane.
It is a further object of this invention to provide an improved process for separating nitrogen and methane employing double column cryogenic rectification in a nitrogen rejection unit which can operate more efficiently at lower available nitrogen concentrations and which can produce some methane at a pressure which exceeds that of the methane as it emerges from the nitrogen rejection unit.